This invention relates to a high performance electrostatic precipitator and more particularly to an electrical system for charging particulates or aerosols in a charging stage of an electrostatic precipitator.
The use of electrostatic precipitators for the collection of particulates is in common practice in industrial settings. These devices have adequate removal efficiency of small particulates, down to 0.1 μm (about 99.9%). They are large and require substantial power. More recently, small scale electrostatic precipitators have been introduced for residential use.
In the United States electrostatic precipitators have had competition from bag houses for the treatment of power plant particulate matter (PM). Bag house units have lower cost of ownership than the electrostatic precipitators (lower capital cost and comparable operating costs), while in Europe for historical reasons electrostatic precipitators are still dominant.
One way to differentiate electrostatic precipitators is whether they use single or two stage precipitators. In a single stage precipitator, both the charging of the particulates and their removal occurs in the same region of the electrostatic precipitator. In the two-stage configuration, charging of the particulates occurs at a different location from their removal.
The use of two stage electrostatic precipitators is found in the literature. See, for example, Myron Robinson in Air Pollution Control, part 1, “Electrostatic Precipitation,” Werner Straus ed., Wiley Interscience pp. 227-335 (1971). The contents of this reference are incorporated herein by reference. The advantage of these devices is that particulate charging in the first stage can be separated from particulate collection in the second stage. Thus each stage can be independently optimized. The charging stage is usually small compared with the collection stage.
The charging stage includes a corona generating element with one polarity, producing ions into a gas stream that drift towards an oppositely charged electrode. Positive corona is preferred in some applications, primarily indoor air cleaning, because of minimization of the production of ozone (important when the clean air is to be used for breathing), but either positive or negative corona can be used.
In order to remove a high fraction of the particulates, it is best if the particulates are charged to the highest level. The drift speed of the particulates in the collection region depends linearly on the charge of the particulates. For particulates >0.2 μm, the particulates charging mechanism is by ion bombardment, which ceases when the local electric fields due to the charges in the particulate oppose the charging electric field, and thus stops the charging process. In this particulate size range the charge in the particulates increases linearly with applied electric field, and thus high electric field in the charging section is as important as high electric field in the collection section.
There are two concerns with the use of conventional electrostatic precipitators. The first one is power requirement and the second is specific volume (size required for accomplishing sufficient PM removal). In addition, for residential use, the production of ozone needs to be minimized.